Radio Frequency (RF) signals and components are used in a variety of devices, including mobile communications devices such as mobile telephones. One type of commonly employed RF component is an RF receiver. Many RF receivers employ a low noise amplifier (LNA) at the front end of the receiver to set a low noise figure for the receiver, thereby enhancing the receiver's ability to receive weak signals.
In some applications, the expected range of received input signal levels is so great that it is desired for stronger signals to bypass the gain of the LNA, for example to prevent saturation in the receiver. Accordingly, LNAs have been developed which include an integral bypass switch which can be selectively engaged when the input signal level is above a certain threshold.
FIG. 1 shows a functional block diagram of an RF switch-around low noise amplifier 100. Switch-around low noise amplifier 100 includes a low noise amplifier (LNA) 110 and a bypass switch 120. An RF input signal 115 is provided to LNA 110 and bypass switch 120, only one of which is selectively enabled at any given time by a mode select signal 125. The outputs of LNA 110 and bypass switch 120 are coupled together to provide an RF output signal 135. When the input level of RF input signal 115 is relatively low, then mode select signal 125 selects and enables LNA 110, and disables bypass switch 120. In that case, LNA 110 amplifies RF input signal 115 to produce RF output signal 135. When the input level of RF input signal 115 is above a certain threshold, then mode select signal 125 selects and enables bypass switch 120, and disables LNA 110. In that case, LNA 110 does not amplify RF input signal 115, but instead RF input signal 115 passes through bypass switch 120 to produce RF output signal 135.
In the past, a switch-around LNA has been realized utilizing depletion mode pHEMT device technology. FIG. 2 shows a schematic diagram of one embodiment of such a switch-around LNA 200. Switch-around LNA 200 includes LNA 210 and bypass switch circuit 220. LNA 210 includes field effect transistor (FET) 201, while bypass switch 220 includes FETs 202, 203 and 204. The FETS in switch-around LNA 200, and particularly amplifier FET 201, are depletion mode pHEMT devices. Switch-around LNA 200 receives power from a first supply voltage VDD, and is further connected to a second supply voltage (e.g., ground).
In operation, an RF input signal 215is provided to LNA 210 and bypass switch circuit 220, only one of which is selectively enabled at any given time by a mode select signal 225. The outputs of LNA 210 and bypass switch circuit 220 are coupled together to provide an RF output signal 235.
It should be noted that mode select signal 225 is produced outside LNA 200 by an external system that evaluates the RF input signal level and in response thereto generates an appropriate mode select signal 225. The external system is not part of LAN 200 proper. In a typical arrangement, this external circuit operates such that when the input level of RF input signal 215 is relatively low (e.g., below a certain threshold), then the external circuit generates a mode select signal 225 having a state that selects and enables LNA 210, and disables bypass switch 220. In that case, LNA 210 amplifies RF input signal 215 to produce RF output signal 235. On the other hand, when the input level of RF input signal 215 is relatively high (e.g., above a certain threshold), then the external circuit generates a mode select signal 225 having a state that selects and enables bypass switch circuit 220, and disables LNA 210. In that case, LNA 210 does not amplify RF input signal 215, but instead RF input signal 215 passes through bypass switch 220 to produce RF output signal 235.
However, switch-around LNA 200 has some shortcomings. First, operation of bypass switch circuit 220 requires an external DC switch 20 that carries the entire current of LNA 210. This level of current is often far higher than what is preferred to be supported by a typical CMOS logic controller that may be employed to implement switch 20. Second, external switch 20 must be bypassed with capacitor 250 to try to effectively restore AC ground to the source of amplifier FET 201. However, it is generally not possible to fully meet the requirement of restoring AC ground to amplifier FET 201 with capacitor 250, as even the best capacitors have an inherent series inductance associated with them that is significant at RF frequencies, and additional trace length to accommodate capacitor 250 adds to the inductance. As a result, the gain of LNA 210 is reduced and, in some cases, the circuit is prone to oscillation.
What is needed, therefore, is an improved switch-around LNA. What is further needed is a switch-around LNA that is more stable against oscillation and provides an improved ground to the active amplifying device of the LNA.